For Summons: Starting at Level 50, Significantly increase the XP needed to level up Summons using the existing formula as a guide. Then at Level 65, increase slightly. Level 80, moderate increase. Level 95, another significant increase, bringing Summons to a crawl.

For Ninjas: Starting at Level 90, slightly increase the XP needed to level up Ninjas using the existing formula as a guide. Then at Level 100, increase slightly. Level 110, increase slightly and so on.

This idea effects everyone equally.

Going forward, % based stats should be added with caution and ALOT of chakra requirements. Ninkame and Katsuyu as well as [ INSERT % based Summon here ] give too much "Bang for your buck" and leave alot of room for premium builds. With the idea above, a small decrease of .25% to these stats could go a long way in balancing Summons.

I will include a very amateur example of what I think a Summon with 1% immunity could look like:

just being obvious here but it doesnt address the imbalance of immu/crit/attack builds in their respective ways and changing the levelling system will amplify the difference between current high level summons/teams and aspiring campers. decreasing the particular stat per level in the summon just prolongs the inevitable(though for spar,making nin immu of kats equal to 0.75 per level will be 100*.75 = 75 nin immu will hurt him, *a little) since there are still custom/stat giving lw's. i know udon has proposed a diminishing returns for immu but has yet to implement it, that would be easier and more effective

Sorry for upping this old post but since the problem still exist and nobody found an idea which could make the community come to a consensus I decided I could give it a try.

From my understand, and after reading this post, and a few others, Immunity is OP but nerfing it drastically would nerf the investment in time and money of a fair amount of players. Also a too brutal nerf would mean replacing a tyranny with another one by making +atk/crit dmg teams OP.

First thing first, imposing a 50% cap to taijutsu immunity, a 50% cap to ninjutsu immunity and a 75% cap on elemental immunity.
Cap crit damage at 100%.

For tai/nin immunity, it would be a rather arbitrary decision but it would match the cap on crit chance and halving a team's potential damage would be devastating enough.
For crit damage it would avoid it being too powerful.
For Elemental immunity, it would avoid having all custom LWs have massive -resist because oh well why not; it doesn't matter anyway.

Now for this system to work, and old katsuyu player to possibly comeback, I would like to see the opportunity to "Transmute summons" rise.
Each summon which core value can obtain a certain "no return point" of diminishing returns would be offered the possibility to be transmuted.
List of core values:
-Tai/Nin immunity (cap 50)
-Crit chance (cap 50)
-Crit Damage (cap 100)
-Reroll (open for discussion)
-Elemental Immu (open for discussion)

The idea would be to compensate the diminishing return of the hard capped core values by an increase in the other stats provided by the summon matching the previous AV/DV.

For example, Sparhawk's katsuyu:
Lvl 100
Nin Up/Immune increased by 500 / 100After conversion to Atk damage, based on Udon's initial calculation of 1% immunity = 1.5 dmg = 3 nin it would give a 250+150=400 AV/DV
If it was to be transmuted, it would come back to lvl 50 (thus matching the core value hard cap) and would see its Nin Up stats increase as to match the "lost" Nin Immu.
Lvl 100
Nin Up/Immune increased by 650 / 50After conversion to Atk damage, based on Udon's initial calculation of 1% immunity = 1.5 dmg = 3 nin it would give a 325+75=400 AV/DV
From this point on, the gross increase (in AV/DV) of the transmuted katsuyu remain the same except the nin Imm would not grow anymore while nin up would grow faster.

Following limitations would apply:
-Can only transmute if summon has a capped value.
-Can only transmute if said value has reached/exceeded its cap.
-Transmuting would incur no extra chakra cost.

[edit] I realized this transmutation of might be posted on its own topic under "Ideas and Suggestions" as it aims at "fixing" much more than immunity.

Looking at the summon list It think the same system could be applied to a handful of other summons :

-Shinigami, may actually make it playable depending on the extra -Enemy Atk down.
-Hachibi, would make it more viable versus SD between level 50 and 100.
-DBB, may make it more represented in the summon ladder. Would raise the issue of how to compensate the loss of CritD; boosting +Atk, +Tai or both? Let the players decide?
-Gamahiro, same as DBB with the possibility of a double transmute?
-Shichibi, would finally make it worth camping with it as it would end up giving tons of endurance after the 2 other stats capped.
-SD, would make it a viable camping summon or at least improve its usefulness after lvl 20...
-Ibuse, Transferring the AV/DV to the ninjutsu may make it too strong while transferring it to the -Immu may make it too weak.
-Panda, indreasing endurance or nin? See how to keep it on par with Ibuse minding the chakra difference.
-Nibi/Conch King, would make them actual camping material.
-Nin kame, same as Katsuyu.

Last but not least, as the chakra cost would not be impacted by the transmutation, setting a fee as high as 10.000 ryo, if not more, would be a nice way to make use of those ryos, especially considering those summons would only/mostly concern camping teams, not progression ones.